Mobile digital television is an emerging technology supported in some mobile computers. One of the standards, DVB-H, short for digital video broadcasting—handheld, is a time-sliced version of DVB-T used for digital terrestrial television. The DVB-T demodulator generates a high-bit-rate MPEG2 transport stream. The transport stream is time-sliced into individual services. For example, a 1 megabit-per-second (Mbps) service for a small display may be generated by time-slicing a 10 Mb transport stream into 100 ms slots every 1 second.
The most important operation in a mobile DVB-H demodulator is estimating the time-varying channel, in other words, accurately estimating the channel frequency response. If this can be done accurately, then it is not that difficult to implement functions like equalization and inter-carrier-interference cancellation. The two known methods for estimating the channel frequency response are to either use the scattered pilots (SPs) of every symbol to do channel estimation or to use time filtering of the SPs.
To reduce power consumption, the tuner and the demodulator are turned on only during the time slice. In practice, it is necessary to allow some time for synchronization and this time has to be minimized to minimize the average power consumption.
Hence, the time available to estimate the channel is very small. This is not a serious issue when the delay spreads of the echo profiles are small, because then a channel estimate may be obtained from the scattered pilots of each symbol. This is not the case for long delay spreads, such as those in single frequency networks.
The scattered pilots method of channel estimation limits the delay spread of the multi-path (i.e. the channel impulse response length) to Tu/12, where Tu is the orthogonal frequency division multiplexing (OFDM) symbol duration. This restriction is unacceptable for single frequency networks designed with typical guard bands of Tu/4 or Tu/8.
To get a reasonable Doppler performance with long delay spreads, as in single frequency networks, it is necessary to use time filtering. In this method, a sliding window is applied over a time sequence of symbols (scattered pilots) to obtain interpolated and filtered versions of the channel estimate. However, note that a scattered pilot pattern repeats every four symbols. Thus, time filtering gives rise to three problems in DVB-H. First, time filtering needs a large memory to store the data symbols, to compensate for the latency of the time filter. This impacts the silicon area and power consumption of a design. Second, time filtering takes more time than the scattered pilots method mentioned above. If the time filter has twelve active taps, then the window size turns out to be about 48 symbols. If the time slice is 100 ms (i.e. 100 8K symbols) then the overhead for time filtering is about 48%, i.e. 48% increase in power consumption. The third problem is the limit on maximum Doppler imposed by the 4-symbol separation of SPs along the time axis. This makes the maximum achievable Doppler 0.125/(Tu+Tg), where Tu and Tg are symbol and guard intervals of the OFDM symbol, respectively. Tu is 896 μs for 8K 8 MHz OFDM. In practice, it is very difficult to get close to this theoretical limit.
Thus, there is a continuing need for an algorithm for mobile DVB-H channel estimation.